Sounds like a problematic heater core.
Sounds like a diesel.
I once had a vehicle come into the shop, a bread truck. They complained of no heat.
We let it idle for hours, even pinched off the radiator hoses with sticks and c-clampa....It ran just fine, but wouldn’t make any significant heat till you took it down the road.
The Dasher had a great heater, as long as there was hot coolant flowing through it.